Preheat your oven to 350°F (175°C), and line two baking sheets with parchment paper so your cookies bake evenly and don’t stick.
In a large bowl, mix the softened salted butter with the brown sugar until smooth and creamy, this sets the stage for that rich, chewy texture.
Add the egg, extra yolk, and vanilla extract, then mix until everything comes together nicely and the mixture smells sweet and buttery.
Whisk the flour, baking soda, and cornstarch in another bowl, then slowly incorporate the dry ingredients into the wet mixture until you get a soft dough.
Scoop the dough into balls, about the size of a golf ball, and place them on your baking sheets, giving them a bit of space.
Gently press each one down, then chill the trays in the fridge for 20 minutes, this helps the cookies bake up thick and chewy.
Bake for around 10 minutes, just until the edges turn golden and the centers look slightly soft. Allow them to rest on the tray for a few minutes to cool.
While they cool, beat the salted butter until fluffy, then mix in the Biscoff spread until smooth.
Add powdered sugar and a little cream at a time until the frosting is thick but spreadable. Once the cookies are cool, frost generously and top with crushed Biscoff cookies for a perfect finish.